Time‐accurate blade surface static pressure behaviour on a rotating H‐Darrieus wind turbine

Time‐accurate blade pressure distributions on a rotating H‐Darrieus wind turbine at representative tip speed ratios during start‐up are presented here, which allow blade dynamic stall and laminar separation bubbles to be observed clearly and which provide a rare experimental demonstration of the flow curvature effect inherent in H‐Darrieus turbine operation. The convection of a dynamic stall vortex along the blade surface at high reduced frequency has also been clearly identified. This study provides new information of the complex aerodynamics of the vertical axis wind turbines (VAWTs) and provides unique experimental data to validate the transient blade static surface pressure distribution predicted by CFD models. To the best of the authors' knowledge, this is the first time that the instantaneous pressure variation around the blade has been measured and recorded directly for an H‐Darrieus wind turbine.     

Experimental Investigation of Aerodynamic Performance due to Blade Tip Clearance in a Gas Turbine Rotor Cascade

This study examines how the complex flow structure within a gas turbine rotor affects aerodynamic loss. An unshrouded linear turbine cascade was built, and velocity and pressure fields were measured using a 5-hole probe. In order to elucidate the effect of tip clearance, the overall aerodynamic loss was evaluated by varying the tip clearance and examining the total pressure field for each case. The tip clearance was varied from 0% to 4.2% of blade span and the chord length based Reynolds number was fixed at 2×10⁵. For the case without tip clearance, a wake downstream of the blade trailing edge is observed, along with hub and tip passage vortices. These flow structures result in profile loss at the center of the blade span, and passage vortex related losses towards the hub and tip. As the tip clearance increases, a tip leakage vortex is formed, and it becomes stronger and eventually alters the tip passage vortex. Because of the interference of the secondary tip leakage flow with the main flow, the streamwise velocity decreases while the total pressure loss increases significantly by tenfold in the last 30% blade span region towards the tip for the 4.2% tip clearance case. It was additionally observed that the overall aerodynamic loss increases linearly with tip clearance.     

The Darrieus wind turbine: Proposal for a new performance prediction model based on CFD

This paper presents a CFD model for the evaluation of energy performance and aerodynamic forces acting on a straight-bladed vertical-axis Darrieus wind turbine. The basic principles which are currently applied to BE-M theory for rotor performance prediction are transferred to the CFD code, allowing the correlation between flow geometric characteristics (such as blade angles of attack) and dynamic quantities (such as rotor torque and blade tangential and normal forces). The model is proposed as a powerful design and optimization tool for the development of new rotor architectures for which test data is not available.After describing and validating the computational model against experimental data, a full campaign of simulation is proposed for a classical NACA 0021 three-bladed rotor.Flow field characteristics are investigated for several values of tip speed ratio, allowing a comparison among rotor operation at optimum and lower C_p values, so that a better understanding of vertical-axis wind turbines basic physics is obtained.     

Simulating the aerodynamic performance and wake dynamics of a vertical‐axis wind turbine

The accurate prediction of the aerodynamics and performance of vertical‐axis wind turbines is essential if their design is to be improved but poses a significant challenge to numerical simulation tools. The cyclic motion of the blades induces large variations in the angle of attack of the blades that can manifest as dynamic stall. In addition, predicting the interaction between the blades and the wake developed by the rotor requires a high‐fidelity representation of the vortical structures within the flow field in which the turbine operates. The aerodynamic performance and wake dynamics of a Darrieus‐type vertical‐axis wind turbine consisting of two straight blades is simulated using Brown's Vorticity Transport Model. The predicted variation with azimuth of the normal and tangential force on the turbine blades compares well with experimental measurements. The interaction between the blades and the vortices that are shed and trailed in previous revolutions of the turbine is shown to have a significant effect on the distribution of aerodynamic loading on the blades. Furthermore, it is suggested that the disagreement between experimental and numerical data that has been presented in previous studies arises because the blade–vortex interactions on the rotor were not modelled with sufficient fidelity. Copyright © 2010 John Wiley & Sons, Ltd.     

Computational fluid dynamics simulation of the aerodynamics of a high solidity,small‐scale vertical axis wind turbine

A computational fluid dynamics simulation was performed for a small‐scale, high solidity (σ = 0.48) H‐type Darrieus vertical axis wind turbine. Two‐dimensional unsteady Reynolds‐averaged Navier–Stokes equations were solved for the turbine numerical model, which has a large stationary domain and smaller rotating subdomain connected by a sliding mesh interface. The simulation results were first validated against steady‐state airfoil data. The model was then used to solve for three rotating blades with constant ambient flow velocity (Re = 360,000) over numerous blade speed ratios. The high solidity and the associated low blade speed ratio and rotational speed of the turbine result in complex flow–blade interaction mechanisms. These include dynamic stall resulting in vortex shedding, vortex impingement on the source blade and significant flow momentum extraction causing reduced power production from the downstream blade pass. Copyright © 2011 John Wiley & Sons, Ltd.     

垂直轴风力机变桨控制策略及气动性能影响研究

以H型Darrieus垂直轴风力机为研究对象,基于不同尖速比下攻角随相位角变化规律,提出一种俯仰角控制策略,即攻角较大时俯仰变化幅值较大,而攻角较小时幅值较小。通过数值计算了解此控制方式对气动性能的影响规律,分析变桨后不同旋转角度下风力机涡量场的变化,并讨论气动载荷变化的原因。结果表明：所提俯仰角控制策略可显著增强风力机功率系数,且尖速比较低时提升效果越显著,在TSR为1.25时功率系数提升高达146%。     

汽轮机排汽缸的气动研究进展

汽轮机排汽缸是连接凝汽式汽轮机末级出口至冷凝器的通道,改善其气动性能可提高汽轮机的效率。本文从数值分析及试验两方面对排汽缸的研究进行了分析,并对提高气动性能的新型措施进行了归纳。分析表明：末级与排汽缸湿蒸气汽液两相流的联合数值分析是排汽缸流场数值分析的发展方向;模型试验是排汽缸研究的主要方法,由于进口气流对排汽缸性能的影响敏感,进口流场应反映末级出口气流的真实状况,并要特别注意叶栅顶部出口气流的模拟。提高排汽缸气动性能的新型措施为：采用具有负超高的扩压器和非对称扩压器;中分面布置轴向栅格型涡阻尼器;采用除湿措施,减少湿度损失;冷凝器喉部结构及加强系统的改进。     

Aero‐structural design optimization of vertical axis wind turbines

In the last decade, vertical axis wind turbines acquired notable interest in the renewable energy field. Different techniques are available to perform aerodynamic and structural simulation of these complex machines, but, to the authors' best knowledge, a comprehensive approach, which includes an automatic optimization algorithm, has never been developed. In this work, a methodology to conduct an efficient aero‐structural design of Darrieus vertical axis wind turbine is presented. This relies on a code‐to‐measurement validated simulation tool based on Blade Element‐Momentum algorithm adopting a particular set of aerodynamic coefficients, and a code‐to‐code validated structural model based on the Euler–Bernoulli beam theory. The algorithms are coupled with a Genetic Algorithm to perform the optimization. The adopted decisional parameters allow to completely vary the blade shape and the airfoil geometry to reduce the structural stress and improve the aerodynamic performance. Different individuals are explored to perform a wide aerodynamic and structural analysis of improved configurations. Copyright © 2016 John Wiley & Sons, Ltd.     

Comparative study of a three-bucket Savonius rotor with a combined three-bucket Savonius–three-bladed Darrieus rotor

The vertical axis wind turbines are simple in construction, self-starting, inexpensive and can accept wind from any direction without orientation. A combined Savonius–Darrieus type vertical axis wind rotor has got many advantages over individual Savonius or individual Darrieus wind rotor, such as better efficiency than Savonius rotor and high starting torque than Darrieus rotor. But works on the combined Savonius–Darrieus wind rotor are very scare. In view of the above, two types of models, one simple Savonius and the other combined Savonius–Darrieus wind rotors were designed and fabricated. The Savonius rotor was a three-bucket system having provisions for overlap variations. The Savonius–Darrieus rotor was a combination of three-bucket Savonius and three-bladed Darrieus rotors with the Savonius placed on top of the Darrieus rotor. The overlap variation was made in the upper part, i.e. the Savonius rotor only. These were tested in a subsonic wind tunnel available in the department. The various parameters namely, power coefficients and torque coefficients were calculated for both overlap and without overlap conditions. From the present investigation, it is seen that with the increase of overlap, the power coefficients start decreasing. The maximum power coefficient of 51% is obtained at no overlap condition. However, while comparing the power coefficients (C_p) for simple Savonius-rotor with that of the combined configuration of Savonius–Darrieus rotor, it is observed that there is a definite improvement in the power coefficient for the combined Savonius–Darrieus rotor without overlap condition. Combined rotor without overlap condition provided an efficiency of 0.51, which is higher than the efficiency of the Savonius rotor at any overlap positions under the same test conditions.     

翼型前缘对风力机翼型气动性能的影响

基于翼型参数化方法对翼型S809进行两类不同的前缘修改,采用翼型设计分析软件Xfoil对修改前、后的翼型进行气动性能计算分析,并采用计算流体力学（CFD）数值模拟方法进行流场特性分析。结果表明：翼型前缘下弯使得翼型在失速区升力系数增大,阻力系数减小,俯仰力矩系数减小,转捩现象延迟,翼型前缘上弯对气动性能的影响与之相反;翼型前缘上弯和下弯使得翼型表面压力系数分布均匀,吸力面及压力面压力系数增大;翼型前缘下弯能够抑制流动分离,抑制涡的形成,延迟翼型失速,翼型前缘上弯对翼型流场特性的影响则与之相反。     

Integrating a new flow separation model and the effects of the vortex shedding for improved dynamic stall predictions using the Beddoes–Leishman method

This paper aims at improving dynamic stall predictions on the S809 aerofoil under 2D flow conditions. The method is based on the well‐known Beddoes–Leishman model; however, a new flow separation model and a noise generator are integrated to improve the predictions in the load fluctuations, including those induced by vortex shedding on the aerofoil upper surface. The flow separation model was derived from a unique approach based on the combined use of unsteady aerodynamic loads measurements, the Beddoes–Leishman model and a trial‐and‐error technique. The new flow separation model and random noise generator were integrated in the Beddoes–Leishman model through a new solution algorithm. The numerical predictions of the unsteady lift and drag coefficients were then compared with the Ohio State University measurements for the oscillating S809 aerofoil at several reduced frequencies and angles of attack. The results using the proposed models showed improved correlation with the experimental data. Hysteresis loops for the aerodynamic coefficients are in good agreement with measurements. Copyright © 2016 John Wiley & Sons, Ltd.     

Development of low-noise drag-type vertical wind turbines

In this study, the aerodynamic noise characteristics of Savonius wind turbines were investigated using hybrid computational aero-acoustics techniques, and low-noise designs were proposed based on the understanding of the noise generation mechanism. First, the flow field around the turbine was analyzed in detail by solving three-dimensional unsteady incompressible Reynolds-averaged Navier–Stokes equations using computational fluid dynamics techniques. Then, the aerodynamic noise radiating from the wind turbine was predicted using the Ffowcs Williams and Hawkings equation with the obtained flow field information. Two distinct harmonic noise components—the blade passing frequency (BPF) and harmonics with a fundamental frequency that is much higher than the BPF—were identified in the predicted noise spectrum. The origin of the higher harmonic components was found to be related to vortex shedding from the rotating turbine. Based on this finding, the proposed low-noise design for Savonius wind turbines uses S-shaped blades. S-shaped blades were found to reduce the noise levels of Savonius wind turbines by up to 2.7 dB.     

Experimental and numerical investigation of tip vortex generation and evolution on horizontal axis wind turbines

The tip vortex of a wind turbine rotor blade is the result of a distribution of aerodynamic loads and circulation over the blade tip. The current knowledge on the generation of the tip vorticity in a 3D rotating environment still lacks detailed experimental evidence, particularly for yawed flow. The aim of this paper is to investigate how circulation at the blade tip behaves and how vorticity is eventually released in the wake, for both axial and 30° yawed flow conditions through the combination of experimental and numerical simulations. Stereo particle image velocimetry is used to measure the flow field at the tip of a 2m diameter, two‐bladed rotor at the TU Delft Open Jet Facility, for both axial and yawed flow; numerical simulations of the experiments are performed using a 3D, unsteady potential flow free‐wake vortex model. The generation mechanisms of the tip vorticity are established. The spanwise circulation along the blade exhibits a similar variation in both axial and yaw cases. A comparison of the chordwise directed circulation variation along the chord between axial and yawed flow is also presented and shown to be different. The analysis is based on contour integration of the velocity field. The tip vortex trajectory for axial flow confirms previous observations on the MEXICO rotor. The experimental results for yawed conditions have clearly shown how vorticity is swept radially away from the blade under the influence of the in‐plane radial component of flow. Such phenomena were only partially captured by the numerical model. The results of this work have important implications on the modelling of blade tip corrections. Copyright © 2015 John Wiley & Sons, Ltd.     

涡轮导向叶栅内流场结构的数值研究

为了详细研究涡轮叶栅的气动特性，深入了解涡轮叶栅流道内的气体流动，对某型涡轮导向叶栅内的流场结构进行了数值模拟。结果表明，对本文研究的叶片弯曲方式，叶片的弯曲能够影响通道涡的位置，但采用弯叶片提高叶栅效率主要是通过降低壁角涡的损失。     

Application of the actuator surface concept to wind turbine rotor aerodynamics

The structure of blade tip vortices is recognized as a key issue in wind turbine aerodynamic modelling by many researchers in the field. In the search for an intermediate model between full Navier–Stokes and blade‐element momentum simulations, this article presents a method using rotating actuator surfaces to model wind turbine aerodynamics. An actuator surface is a simple planar surface, porous to the flow, which is characterized by velocity and pressure discontinuities, whose action on the flow is achieved through an attached system of forces. These discontinuities and forces are determined from blade‐element analysis and the Kutta–Joukowski relation. After implementing this concept in a three‐dimensional CFD (Computational Fluid Dynamics) method, results are produced for the experimental rotors of NREL and TUDelft. The method is validated against both experimental measurements and the predictions of three other numerical models for wind turbine aerodynamic analysis. Qualitative and quantitative comparisons show that the actuator surface concept agrees well with the other numerical models. In addition to rotor aerodynamic analysis, the actuator surface concept can be used in the study of wake aerodynamics, or as the Eulerian flow solver in hybrid methods. Copyright © 2009 John Wiley & Sons, Ltd.     

定常吸气对垂直轴风力机性能影响研究

定常吸气装置可有效提高垂直轴风力机气动性能,改善风轮流场结构及翼型动态失速特性.基于CFD方法对垂直轴风力机进行数值模拟,研究不同叶尖速比(TSR)下定常吸气对风力机气动及流场特性的影响,对比分析原始风力机及定常吸气作用下的风能利用率、整机转矩系数及涡量分布.结果表明:不同尖速比下定常吸气均可显著提高风力机气动性能,减...     

燃气轮机涡轮盘腔燃气入侵与封严技术研究进展

涡轮轮缘间隙燃气入侵及封严技术是燃气轮机领域研究的热点和难点问题。本文针对转-静盘腔内部流动及轮缘密封、多物理场环境下热运行轮缘间隙、封严扫吹流与主流相互作用进行综述,分析并总结了燃气入侵与封严技术的未来研究重点和发展趋势,包含非定常多模态高温燃气入侵理论、轮缘间隙大尺度涡系结构演化规律及封严扫吹流径向迁移机制及损失机理等。     

吹气控制策略对垂直轴风力机气动性能的影响

为改善垂直轴风力机周围流场结构并提升气动性能,在风力机叶片中采用外吹式流动控制,并提出5种吹气控制策略,通过数值模拟的方法研究不同吹气控制策略对垂直轴风力机气动性能的影响,进一步分析在最佳吹气流动控制策略时的涡量场与载荷波动。研究表明:采用上开口抛物线控制策略时的风力机气动性能最佳,当最大吹气动量系数为0. 025时,风能利用系数及平均力矩系数提升26%,并可抑制大涡的形成及发展,同时改善翼型表面压力分布。     

垂直轴风力机动态流场及其气动性能分析

垂直轴风力机气动性能研究是风力机设计、实验的重要部分,对其运动状态下的流场进行分析是观测垂直轴风力机性能重要环节.基于NACA0012对称翼型,建立二维几何模型并进行模拟计算.采用k-ωSST湍流模型及滑移网格技术,通过CFD软件数值计算得到达里厄型直叶片垂直轴风力机运行时周边流场分布情况.通过比较不同方位角下流场涡量以及升、阻力系数得出:在方位角为105°附近时,翼型下表面产生流动分离,并导致失速;下风区翼型运行的流场由于受到上风区尾流的影响,翼型周围没有产生明显的流动分离.